JP2006207003A - Method for recovering noble metal from catalyst layer constituting electrode of fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for recovering a noble metal, which is a catalyst, from a catalyst layer of a used fuel cell with high efficiency at high purity and by a simple operation. <P>SOLUTION: The catalyst layer is taken out of a membrane-electrode assembly of the fuel cell and after the catalyst layer is pulverized, the powder is put into an electrodeposition liquid composed of a polar solvent (for example, acetonitrile) and a basic compound (for example, triethylamine) and is subjected to electrophoresis. Carbon powder is ionized and is deposited on the electrode 2. The noble metal is recovered by a means, such as filtration, from the electrodeposition liquid separated from the carbon powder. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for recovering a noble metal from a catalyst layer constituting an electrode of a fuel cell, and in particular, from an electrode catalyst layer constituting a membrane electrode assembly used in a polymer electrolyte fuel cell, an expensive functioning as a catalyst. The present invention relates to a method for recovering precious metals.

  Fuel cells are entering the actual machine stage from the experimental stage, and there are great expectations as an environmentally friendly energy source. A fuel cell has a basic unit in which an electrode that is an air electrode and an oxygen electrode is located with an electrolyte interposed therebetween. In a polymer electrolyte fuel cell, a solid ion exchange membrane (for example, a perfluoro membrane) is made of PTFE (polyethylene oxide). Tetrafluoroethylene) fibrils integrated in the form of a membrane are used as the electrolyte membrane, and as the electrode, carbon powder carrying precious metal fine particles (platinum, ruthenium, etc.) as a catalyst is applied by a wet method or a dry method. What is laminated on the electrolyte membrane is used, and is called an electrode catalyst layer or simply a catalyst layer. And what laminated | stacked the said catalyst layer which comprises an electrode on both sides of an electrolyte membrane is called a membrane electrode laminated body (MEA: Membrane-Electrode Assembly), and a diffusion layer is usually laminated | stacked on the outer side of a catalyst layer. Is done.

  Precious metals that function as catalysts in the catalyst layer are expensive, and at the stage where fuel cells are widely used as actual equipment, it is also possible to recover the precious metal from the used catalyst layer and reuse it from the viewpoint of cost. This is an urgent issue in terms of environmental protection. As an answer to that problem, Patent Document 1 discloses that the carbon carrying platinum is completely removed by pulverizing the used electrode and collecting it in a required combustor, and then applying heat from the outside of the heating vessel. A method is described in which platinum is burned and directly dissolved from a combustion residue or dissolved in an inorganic acid (aqua regia, sulfuric acid) to recover platinum. A method of converting platinum into a volatile compound by burning a used electrode in an oxidizing gas (chlorine, carbon dioxide) is also performed.

JP 63-161129 A

  In general, in the case of the combustion method, a special combustor and a combustion environment are required, and the work is complicated. In addition, the method of converting platinum into a volatile compound requires other compounds (phosphorus pentachloride, magnesium perchlorate, calcium chloride) to react platinum with an oxidizing gas. And exhaust treatment of by-products is also required. In the method of dissolving in aqua regia etc., the membrane electrode assembly usually contains PTFE as described above, and the presence of PTFE inhibits the extraction action by acid. In addition, a large amount of acid is required to dissolve platinum, and there are problems such as low concentration (low efficiency) recovery and difficulty in separating dissolved platinum ions from other metal ions. .

  The present invention has been made in view of the above circumstances, and recovers a noble metal, which is a catalyst, with high purity from a catalyst layer constituting an electrode of a fuel cell with high efficiency and simple operation. It aims to provide a completely new collection method that makes it possible.

  In order to solve the above problems, the present invention basically uses the principle of electrophoresis. That is, the present inventors pay attention to the fact that functional groups (-COOH, -OH, etc.) generally exist on the surface of carbon powder, and ionization (charging) is performed by removing hydrogen from the functional group using a basic compound. For example, focusing on the fact that only carbon powder can be drawn to the collection electrode side by electrophoresis, the present invention has been made.

  That is, the present invention is a method for recovering a noble metal as a catalyst from a catalyst layer constituting an electrode of a fuel cell, wherein a mixture of the noble metal constituting the catalyst layer and the carbon powder supporting the noble metal is treated with a polar solvent and a basic It is characterized by ionizing carbon powder in an electrodeposition solution composed of a compound, depositing and separating carbon powder on an electrode by electrophoresis, and recovering noble metal from the electrodeposition solution separated by carbon powder And

  In the present invention, the noble metal is separated and recovered by a physical method from the electrodeposition liquid from which the carbon powder has been separated by an electrochemical method, and the precious metal is not dissolved. Precious metal recovery with high purity and high efficiency is possible. Further, since no combustion process is performed, an exhaust process or the like is unnecessary, and the entire recovery process is simplified.

  In the present invention, the electrophoresis method itself may be a conventionally known electrophoresis method and is not special. Copper, nickel, silver, platinum, or the like can be used as appropriate for the electrode for collecting carbon powder and the counter electrode. The electrodeposition liquid may also be an electrodeposition liquid used in a conventional electrophoresis method including a polar solvent on the condition that a basic compound is included as described later.

  The present invention is characterized in that the electrodeposition liquid contains a basic compound in order to ionize the carbon powder (to give an electric charge), and the surface of the carbon powder is caused by the presence of the basic compound in the electrodeposition liquid. Hydrogen is released from the functional group located at, and the carbon powder is ionized. The ionized carbon powder migrates in the electrodeposition liquid, adheres to the electrode surface for collecting the carbon powder, and is separated from the electrodeposition liquid. On the other hand, the noble metal peeled off from the surface of the carbon powder is not ionized, so it stays in the electrodeposition liquid and becomes a precipitate. By precipitating the precipitated noble metal together with the electrodeposition liquid from the apparatus and separating it by an appropriate physical means such as filtration, the noble metal can be recovered with high purity and high efficiency. The electrodeposition liquid can be used again for the next separation and recovery by adding the basic compound reduced by ionizing the carbon powder and the polar solvent evaporated.

  In the present invention, preferably, ultrasonic vibration is applied to the electrodeposition liquid during electrophoresis. As a result, the dispersion of the mixed solution of the electrodeposition liquid in a slurry state and the noble metal-supported carbon powder and the peeling of the supported noble metal from the surface of the carbon powder are facilitated, and the carbon powder by electrophoresis in a short time is promoted. Precipitation separation on the electrode becomes possible.

  In the present invention, when recovering the noble metal in the catalyst layer from the spent fuel cell, as a previous step, a step of taking out the membrane electrode assembly from the fuel cell, a catalyst layer and an electrolyte from the taken out membrane electrode assembly The step of taking out the laminate with the membrane, the step of immersing the laminate of the catalyst layer and the electrolyte membrane in the alcohol solution and dissolving the electrolyte in the electrolyte membrane and other electrolytes in the alcohol, the catalyst from the alcohol solution in which the electrolyte is dissolved And a step of taking out a mixture of noble metal constituting the layer and carbon powder supporting the noble metal.

  In a normal case, the fuel cell module is disassembled and the separator is separated into a single membrane electrode assembly. Further, the diffusion layer is peeled off from the membrane electrode assembly to obtain a laminate of the catalyst layer and the electrolyte membrane. Next, the electrolyte membrane and the catalyst layer (pulverized product thereof) are immersed in alcohol, preferably while pulverizing them and stirring ultrasonically in a heated and pressurized state. Thereby, the electrolyte in the electrolyte membrane and the electrolyte that may be present in the catalyst layer are dissolved in the alcohol. Thereafter, the insoluble matter (PTFE used to reinforce the electrolyte membrane, peeled noble metal, carbon powder supporting the noble metal) is collected by means such as filtering or centrifuging the alcohol solution in which the electrolyte is dissolved. . Then, the recovered material is put into an electrophoresis apparatus, and the carbon powder and the noble metal are separated and recovered as described above.

  In the present invention, polar solvents include nitrile compounds such as acetonitrile, propionitrile, butyl nitrile, isabutyl nitrile and benzonitrile, sulfur-containing compounds such as dimethyl sulfoxide, sulfolane and dimethylthioformamide, N-methylpyrrole. A group consisting of nitrogen-containing compounds such as drin, pyridine and diviridine, ester compounds such as propylene carbonate, dimethyl carbonate, diethyl carbonate and trimethyl phosphoric acid, alcohols such as ethyl alcohol and 2-propanol, and lactones such as butyrolactone. The at least 1 sort (s) chosen from can be mentioned. Particularly effective solvents are acetonitrile, propylene carbonate, and butyrolactone as main components from the viewpoints of electrochemical stability (because they are not electrolyzed) and viscosity (to ensure smooth ion diffusion and conductivity). .

In the present invention, the basic compound is represented by the following formula 1

The compound which has the functional group containing the at least 1 sort (s) of element normally selected from the group which consists of nitrogen, phosphorus, and sulfur which can form onium salt shown by these is mentioned.

  In Formula 1, Z represents a nitrogen or phosphorus atom. R1, R2, and R3 represent the same or different organic groups having 1 to 14 carbon atoms.

  R1 and R2, or R1, R2 and R3 may be combined to form a heterocyclic group together with the nitrogen atom, phosphorus atom or sulfur atom to which they are bonded.

  Examples of the organic group having 1 to 14 carbon atoms represented by R1, R2, and R3 include, for example, a hydroxyl group, an alkoxyl group, an ether bond, and the like, which may contain a different atom such as an oxygen atom. -14 Preferably a C1-C8 hydrocarbon group is mentioned. Examples of the hydrocarbon group include aliphatic, alicyclic or aromatic hydrocarbon groups such as an alkyl group, a cycloalkyl group, a cycloalkylalkyl group, an aryl group, and an aralkyl group.

  Of these, an alkyl group is particularly preferred. The alkyl group may be linear or branched, and specifically includes, for example, methyl, ethyl, n- or isopropyl, n-, iso-, sec- or tert-butyl, pentyl , Heptyl, octyl group and the like.

  As said cycloalkyl group or a cycloalkylalkyl group, a C5-C8 thing is preferable, and a cyclopentyl, a cyclohexyl, a cyclohexylmethyl, a cyclohexylethyl group etc. are mentioned specifically ,.

  Specific examples of the aryl group include phenyl, toluyl, and xylyl groups.

  Specific examples of the aralkyl group include benzyl and phenethyl groups.

  Preferred examples of the hydrocarbon group containing a hetero atom such as an oxygen atom include hydroxyalkyl groups, specifically hydroxymethyl, hydroxyethyl, hydroxybutyl, hydroxypentyl, hydroxyhexyl, hydroxyheptyl, hydroxyoctyl. Examples thereof include an alkoxyl group, specifically a methoxymethyl, ethoxymethyl, ethoxyethyl, n-propoxyethyl, iso-propoxymethyl, n-butoxymethyl, iso-butoxyethyl, tert-butoxyethyl group and the like.

In addition, as a group in the case where R1 and R2, or R1, R2 and R3 are combined to form a heterocyclic group together with the nitrogen atom, phosphorus atom or sulfur atom to which they are bonded, specifically, The following formula 2 can be exemplified.

  In particular, it is preferable that R1, R2 and R3 are alkyl groups having 1 to 2 carbon atoms because (a) high ion separation can be expected and (b) high ion conduction (mobility) can be expected. The best are ethylmethylamine, trimethylamine, ethyldimethylamine, diethylmethylamine.

  Hereinafter, the present invention will be described based on embodiments with reference to the drawings. FIG. 1 is a diagram for explaining an example of a polymer electrolyte fuel cell, and FIG. 2 shows a process from dismantling the fuel cell module to recovering noble metal therefrom. FIG. 3 shows an example of an electrophoresis apparatus used in the noble metal recovery method according to the present invention, and FIG. 4 is a schematic view showing an embodiment in which carbon powder is ionized in the electrodeposition liquid.

  A polymer electrolyte fuel cell is known as one of the fuel cells. As shown in FIG. 1, a number of fuel cell modules 10 are assembled in an airtight manner to form a fuel cell stack 11, which is put to practical use. The fuel cell module 10 includes a membrane electrode assembly 5 in which a catalyst layer 2 and a diffusion layer 3 are stacked on both sides of an electrolyte membrane 1 as electrodes (anode side electrode, cathode side electrode) 4, and ribs for sandwiching the membrane electrode assembly 5 from both sides (Gas flow path) It is normally comprised with the separator 7 provided with 6 and 6. FIG. Holes (manifold holes) 8... Serving as gas flow paths are formed in the outer peripheral region of the separator 7, and fuel gas (hydrogen) and oxidizing gas (oxygen, usually air) necessary for the operation of the fuel cell are provided in the manifold holes 8. .. Are supplied to the electrodes 4 and 4 via ribs (gas flow paths) 6 and 6 formed on both sides of the separator 7. A manifold hole for cooling water may also be provided.

  The present invention will be described by taking, as an example, the case where the spent fuel cell stack 11 of the above-described form is disassembled and the noble metal (here, platinum) is recovered from the catalyst layer 2. FIG. 2 shows the process. First, the fuel cell stack 11 is disassembled into a number of fuel cell modules 10, and each module 10 is disassembled into separator waste 7 and a membrane electrode assembly (MEA) 5 ( S201). The diffusion layer 3 is peeled off from the MEA 5 to obtain a laminate of the electrolyte membrane 1 and the catalyst layer 2 (S202).

  After the laminated body of the electrolyte membrane 1 and the catalyst layer 2 is physically pulverized in a pulverizer, it is immersed in a container containing alcohol and subjected to ultrasonic stirring (S203). Thereby, alcohol-soluble substances (an electrolyte that is a part of the electrolyte membrane 1 and an electrolyte that may constitute a part of the catalyst layer 2) are dissolved in the alcohol. The carbon powder supporting PTFE and platinum used in the electrolyte membrane 1 is insoluble in alcohol and remains as it is. By this ultrasonic agitation, a part of platinum carried on the carbon powder may be peeled off from the carbon powder.

  The slurry-like alcohol solution containing the alcohol-insoluble solid is centrifuged (S204). Thereby, the peeled platinum which is an insoluble matter, the carbon powder carrying platinum and the PTFE are separated from the alcohol solution in which the electrolyte is dissolved. Further, PTFE is separated from the insoluble material by sieving.

  The remaining peeled platinum and carbon powder carrying platinum are put into an electrophoresis apparatus. FIG. 3 shows an example of the electrophoretic apparatus 20. In the electrolytic cell 21, a carbon recovery electrode 23 and a counter electrode 24 are placed opposite to each other with an electrodeposition liquid isolation film 22 made of, for example, porous ceramics interposed therebetween. A material such as copper, nickel, or silver is used for the carbon recovery electrode 23, and a material such as platinum is used for the counter electrode 24. In the illustrated example, the carbon recovery electrode 23 has a strip shape and a circulation type in order to facilitate the continuous recovery of the carbon powder deposited on the electrode, but it may be a fixed electrode. A voltage from the power source 25 is applied between the carbon recovery electrode 23 and the counter electrode 24 to apply an electric field. An ultrasonic generator 26 is disposed in the vicinity of the carbon recovery electrode 23 in the electrolytic cell 21.

  As described above, the electrodeposition liquid 27 in the electrolytic cell 21 is preferably a polar solvent such as acetonitrile, propylene carbonate, or butyrocraton, and a basic compound such as ethyl methyl humin, triethyl amine, ethyl dimethyl amine, or diethyl methyl amine. It is included. The amount of the basic compound may be at least an amount sufficient to ionize the carbon powder to be added, and can be appropriately supplemented.

  The separated platinum after separation and the carbon powder carrying platinum are put into the carbon recovery electrode 23 side of the electrolytic cell 21. While applying the electric field or before applying the electric field, the ultrasonic generator 26 is operated to apply ultrasonic vibration to the electrolytic solution 27. Thereby, peeling of platinum from the surface of the carbon powder proceeds more reliably. On the other hand, as shown in the schematic diagram of FIG. 4, a functional group (in this example, —COOH) exists on the surface of the carbon powder 31, and a basic compound (in this example, an amine) becomes a functional group hydrogen ion. The carbon powder 31 is ionized by coordination to become an ammonium cation.

  By applying an electric field in this state, the ionized carbon powder is electrostatically attracted to the carbon recovery electrode 23, and is electrodeposited and deposited on the electrode. On the other hand, the peeled platinum is not subjected to the action of a basic compound (amine in this example), and therefore precipitates on the bottom of the electrodeposition tank 21 when the ultrasonic wave is stopped. By taking out together with the electrodeposition liquid 27 and separating it into means such as filtration, platinum can be reliably recovered.

  Hereinafter, the present invention will be described with reference to examples and comparative examples.

[Example 1]
The electrophoresis apparatus 20 shown in FIG. 3 was used. A copper foil (thickness 50 μm) was used as the carbon recovery electrode 23, and a platinum electrode (thickness 100 μm) was used as the counter electrode 24. Acetonitrile was placed in the tank 21 on the counter electrode 24 side. On the other hand, acetonitrile and triethylamine were stirred in the tank 21 on the carbon recovery electrode 23 side, and 440 mg of a mixture of peeled platinum recovered from the membrane electrode assembly (MEA) and carbon powder carrying platinum was put therein, The electrolytic solution in a slurry state for 30 minutes was stirred and dispersed with ultrasonic waves. The electrophoresis conditions were an applied voltage of 400 V and an energization time of 10 minutes.

  After 10 minutes, when the copper electrode (carbon recovery electrode 23) was pulled up, the electrode surface was covered with carbon powder, and the platinum powder was sinking to the bottom of the tank. The platinum powder was extracted together with the electrodeposition solution and filtered to recover 200 mg of platinum powder. Colorimetric analysis was performed on the ash remaining after burning the recovered carbon on the copper foil, but platinum was below the lower limit of detection. There was no platinum detection from the electrodeposition solution after filtration, and the platinum recovery rate was 100%.

  The electrodeposition solution could be used repeatedly by adding reduced amounts of triethylamine and evaporated solvent.

[Example 2]
Fractionation was performed under the same conditions as in Example 1 except that acetonitrile and methyldiethylamine were placed in the tank 21 on the carbon recovery electrode 23 side. Similarly, carbon powder covered the copper electrode surface, and platinum powder was sinking to the bottom of the tank. The platinum powder was extracted together with the electrodeposition solution and filtered to recover the platinum powder. The platinum recovery rate was 98.9%. In this case as well, the electrodeposition solution could be used repeatedly by adding the reduced amount of methyldiethylamine and the evaporated solvent.

[Comparative Example 1]
The same amount of platinum-supported carbon powder as in Example 1 was placed in aqua regia and ultrasonically stirred for 60 minutes while heating to 90 ° C. Thereafter, the carbon powder was filtered off with a PTFE filter. Platinum dissolved in aqua regia diluted 100 times was analyzed by atomic absorption spectrophotometry, and it was found that the platinum weight in the aqua regia solution was 194 mg. The aqua regia was dried and hydrochloric acid was added, and the denitration operation was repeated 2-3 times, diluted, cooled and filtered to obtain a hydrochloric acid solution in which platinum was dissolved. In this hydrochloric acid solution, 189 g of platinum reduced with sodium borohydride was recovered. The recovery rate was 94.5%.

[Comparative Example 2]
The same operation as in Comparative Example 1 was performed to obtain an aqua regia solution in which platinum was dissolved. This aqua regia solution was transferred to the electrolytic layer and electrolyzed with a current of 30 A to recover 183 mg of platinum. The recovery rate was 91.5%.

[Discussion]
In the recovery method according to the present invention, a recovery rate as high as 100% or 98.9% could be achieved. On the other hand, Comparative Examples 1 and 2 require many steps and require a long treatment time, and the recovery rate is as low as 94.4% or 91.5%, indicating the superiority of the present invention.

The figure explaining an example of a polymer electrolyte fuel cell. The figure which shows the process after dismantling a fuel cell module and collect | recovering noble metals from there. The figure which shows an example of the electrophoresis apparatus used with the collection | recovery method of the noble metal by this invention. The schematic diagram which shows the aspect which carbon powder ionizes in an electrodeposition liquid.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 ... Fuel cell module, 1 ... Electrolyte membrane, 2 ... Catalyst layer, 3 ... Diffusion layer, 4 ... Electrode (anode side electrode, cathode side electrode), 5 ... Membrane electrode assembly, 7 ... Separator, 20 ... Electrophoresis apparatus , 21 ... Electrolyzer, 22 ... Electrodeposition separator, 23 ... Electrode for carbon recovery, 24 ... Counter electrode, 25 ... Power source, 26 ... Ultrasonic generator, 27 ... Electrodeposition solution

Claims (5)

  A method for recovering a noble metal as a catalyst from a catalyst layer constituting an electrode of a fuel cell, wherein a mixture of the noble metal constituting the catalyst layer and a carbon powder supporting the noble metal is electrodeposited containing a polar solvent and a basic compound Precious metal from the catalyst layer, characterized by ionizing carbon powder in a liquid, ionizing and separating the carbon powder on an electrode by electrophoresis, and recovering the precious metal from the electrodeposition liquid from which the carbon powder has been separated Collection method.   2. The method for recovering a noble metal from a catalyst layer according to claim 1, wherein ultrasonic vibration is applied to the electrodeposition liquid.   As a previous step, a step of taking out the membrane electrode assembly from the fuel cell, a step of taking out the laminate of the catalyst layer and the electrolyte membrane from the taken out membrane electrode assembly, and immersing the laminate of the catalyst layer and the electrolyte membrane in an alcohol liquid The method further comprises: a step of dissolving the electrolyte, and a step of taking out a mixture of the noble metal constituting the catalyst layer and the carbon powder supporting the noble metal from the alcohol solution in which the electrolyte is dissolved. For recovering precious metals from the catalyst layer.   The noble metal from the catalyst layer according to any one of claims 1 to 3, wherein the basic compound is at least one selected from the group consisting of ethylmethylhumin, triethylamine, ethyldimethylamine or diethylmethylamine. Collection method.   The method for recovering a noble metal from a catalyst layer according to any one of claims 1 to 3, wherein the polar solvent is at least one selected from the group consisting of acetonitrile, propylene carbonate, and butyrocraton.

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